A silica glass crucible is used in the manufacture of a silicon single crystal. In a Czochralski (CZ) method, polysilicon is charged into a silica glass crucible, heated and melted and a seed crystal is immersed in this silicon melt, and while mutually rotating the crucible and the seed crystal in opposite directions, the seed crystal is slowly pulled up and a silicon single crystal is grown. In order to manufacture a high purity silicon single crystal for use in semiconductor devices, it is necessary that the silicon single crystal not be polluted by an elution of the silica glass crucible and in addition, the silica glass crucible requires sufficient heat resistance strength.
There is natural silica and synthetic silica in the raw material of the silica glass crucible and generally natural silica has a lower level of purity than synthetic silica but has excellent heat resistance strength whereas synthetic silica has poor heat resistance strength but a high level of purity. Thus, a silica glass crucible having a two layered structure including forming an outer layer of the crucible with natural silica and increasing the strength of the crucible under a high temperature and forming an inner layer of the crucible which contacts with the silicon melt with synthetic silica and which prevents incorporating impurities, is generally used (refer to Japanese Patent Application Laid Open No. H01-261293). In addition, a crucible which has an inner layer with essentially no bubbles is also known (refer to Japanese Patent Application Laid Open No. H02-55285).
The weight of the silicon which is charged into a crucible is increasing due to the large scale of silicon ingots in recent years. As a result, it becomes more difficult to remove gas bubbles which are included within the silicon melt and these gas bubbles are incorporated into the silicon single crystal during growth causing cavity defects (also called voids or air pockets) which are formed within the crystal. Such problems have become noticeable. Argon (Ar) gas which is attached to the inner surface of the silica glass crucible, and silicon monoxide (SiO) gas which is produced by a reaction between the silica glass crucible and silicon melt are known as causes of the cavity defects. The cavity defects caused by gas bubbles largely have a spherical shape with a diameter of 300-500 μm taking up the majority of its size. However, very small cavity defects with a diameter of 150 μm or less and very large cavity defects with a diameter of 1 mm or more are also sometimes formed. In this way, cavity defects caused by gas bubbles clearly have different characteristics to Grown-in defects such as COP (Crystal Originated Particle). Presently, the presence of these defects can not be nondestructively inspected. The cavity defects can be detected only after a wafer has been cut from the ingot and appear as through holes or pinholes on the surface or interior section of the wafer.
In recent years, the effects on semiconductor devices by pinholes with a wafer are extremely large. The effects of pinholes differ depending on the size, number, position of generation and type of semiconductor device. However, in the latest semiconductor devices which have a very high integration, such effects can not be ignored and either elements can not be formed in regions which include cavity defects or even when elements can be formed their characteristics become significantly poor or unstable. In particular, because this results in many defective products and because yield of the semiconductor device decreases significantly when the number of pinholes within a wafer is large, the wafer itself has to be discarded. Therefore, it is necessary to reduce the possibility of pinholes being included within a wafer to almost zero.
In order to solves this problem, a method of adjusting a furnace pressure when melting polysilicon is proposed, for example, in Japanese Patent Application Laid Open Nos. H05-9097 and 2000-169287. In addition, a method of providing vibration to a crucible and starting growth of a silicon single crystal after reducing gas bubbles which are attached to the inner surface of the crucible is proposed in Japanese Patent Application Laid Open No. 2007-210803.
However, an environment for preventing generation of gas bubbles described above and a process for removing gas bubbles are not sufficient for manufacturing a high quality silicon single crystal without cavity defects caused by gas bubbles. Ingenuity for actively removing gas bubbles from a silicon melt is required.